Why I use reverse mouselook - in a word, flying games.
The default movement for joysticks was always push forward to dip the nose, and down to climb. Since Descent (1/2/freespace) and EF2000 (and Total Air War) were some of the first games I played, anytime I'm in a "3d" environment I want the mouse to behave that way. If they ever take away that option from games I'll be in big, big trouble.